Various types of containers currently made from glass are being replaced by plastic containers due to the weight, bulkiness, and susceptibility to breakage inherent in glass containers. In many cases, these containers can be manufactured from existing polymers, such as the polyesters described in U.S. Pat. No. 4,983,711. Polyvinylchloride (PVC) and polycarbonate (PC) are other materials often used for extrusion blow-molded containers. In certain circumstances, however, these polymers do not meet fitness-for-use criteria when used in their neat form. For example, when the containers must contain liquids hotter than 75° C., the polyesters described in the '711 patent and PVC are not adequate due to low softening points. Similarly, polycarbonate is often unacceptable in the same applications due to poor chemical resistance to the contents or cleaners used during processing of the bottles. In addition, polycarbonate often requires complicated annealing procedures to remove residual stresses formed during processing.
In order to take advantage of selected properties of different polymers, for example, high temperature resistance and good chemical resistance, they are often blended together. However, not all blends are transparent; thus, the selection of materials that can be blended together is further limited by the need to create transparent containers.
Blends of polycarbonate and certain polyesters are used in injection molding and sheet extrusion applications. These blends are clear and can provide a good balance of chemical resistance and heat resistance. However, existing commercial transparent blends of polycarbonate and polyesters cannot be processed by extrusion blow-molding due to lack of melt strength.
Manufacturing equipment and processes have been designed and put into use for the cost-efficient production of various types and sizes of containers at high rates. One of these manufacturing processes is extrusion blow-molding wherein a polymer melt is extruded from a die downward in the shape of a hollow cylinder or tube. Bottles and other shaped articles are produced by clamping a mold around the molten, hollow cylinder and injecting a gas, e.g., air, into the molded-encased cylinder to force the molten polymer into the mold. For a polymer to be useful in extrusion blow-molding processes, the polymer should possess sufficient melt strength. To be useful for the production of rigid (self-supporting) containers, especially relatively large containers, e.g., containers intended for packaging volumes of 3 L or greater, and containers having an irregular shape, the polymer should also possess adequate physical, tensile, and thermal properties.
Many polymeric materials do not possess melt strength sufficient to render them suitable for extrusion blow-molding, and when extruded downward from a die, the polymer melt drops rapidly and forms a thin string and/or breaks. Polymers suitable for extrusion blow-molding have a melt strength that is sufficient to support the weight of the polymer. Good melt strength is desired for the manufacture by extrusion blow-molding of containers having uniform wall thickness.
Since melt strength is related to slow flow, which is induced primarily by gravity, melt strength can be related to the viscosity of a polymer measured at a low shear rate, such as 1 radian/second. Viscosity can be measured by typical viscometers, such as a parallel plate viscometer. Typically, viscosity is measured at the typical processing temperature for the polymer and is measured at a series of shear rates, often between 1 radian/second and 400 radian/second. In extrusion blow-molding, the viscosity at 1 radian/second at processing temperatures typically needs to be above 30,000 poise in order to blow a bottle. Larger parisons require higher viscosities.
Melt strength, however, only defines one of the polymer processing characteristics desired in extrusion blow-molding. Another desired characteristic is the ease of flow at high shear rates. The polymer is “melt processed” at shear rates ranging anywhere from about 10 s−1 to 1000 s−1 in the die/extruder. A typical shear rate encountered in the barrel or die during extrusion blow-molding or profile extrusion is 400 radians/second. These high shear rates are encountered as the polymer flows down the extruder screw, or as it passes through the die. These high shear rates are desired to maintain reasonably fast production rates. Unfortunately, high melt viscosity at high shear rates can lead to viscous dissipation of heat, in a process referred to as shear heating. Shear heating raises the temperature of the polymer, and the extent of temperature rise is directly proportional to the viscosity at that shear rate. Since viscosity decreases with increasing temperature, shear heating decreases the low shear rate viscosity of the polymer, and thus, its melt strength decreases.
Furthermore, a high viscosity at high shear rates (for example, as found in the die) can create a condition known as melt fracture or “sharkskin” on the surface of the extruded part or article. Melt fracture is a flow instability phenomenon occurring during extrusion of thermoplastic polymers at the fabrication surface/polymer melt boundary. The occurrence of melt fracture produces severe surface irregularities in the extrudate as it emerges from the orifice. The naked eye detects this surface roughness in the melt-fractured sample as a frosty appearance or matte finish as opposed to an extrudate without melt fracture that appears clear. Melt fracture can occur whenever the wall shear stress in the die exceeds a certain value, typically 0.1 to 0.2 MPa. The wall shear stress is directly related to the volume throughput or line speed (which dictates the shear rate) and the viscosity of the polymer melt. By reducing either the line speed or the viscosity at high shear rates, the wall shear stress is reduced, lowering the possibility for melt fracture to occur. Although the exact shear rate at the die wall is a function of the extruder output and the geometry and finish of the tooling, a typical shear rate that is associated with the onset of melt fracture is 400 radian/sec. Likewise, the viscosity at this shear rate typically needs to be below 10,000 poise.
To couple all of these desired properties, the ideal extrusion blow-molding polymer, from a processability standpoint, will possess a high viscosity at low shear rates in conjunction with a low viscosity at high shear rates. Fortunately, most polymers naturally exhibit at least some degree of viscosity reduction between low and high shear rates, known as “shear thinning”, which aids in their processability. Based on the preceding discussion, one definition of shear thinning relevant to extrusion blow-molding would be the ratio of the viscosity measured at 1 radian/second to the viscosity measured at 400 radians/second when both viscosities are measured at the same temperature. The measurement temperature selected should be typical of the actual processing conditions and one that provides a viscosity of 10,000 poise or less at 400 rad/sec. This definition will be used to describe shear thinning for the purposes of this invention. Based on the preceding discussion, a good extrusion blow-molding material would possess a shear thinning ratio of 3.0 or higher when measured at a temperature that provides a viscosity at 400 rad/sec of 10,000 poise or less.